A demand for a high density optical disk with a memory capacity equal to or larger than 20 GB has been increasing for storing image data used in High-Definition TV (HDTV) broad casting and the like. To meet this demand various techniques have been reported from the companies concerned, wherein an optical disk is included called a Blue-ray Disk (BD). A DVD-R/RW or DVD-RAM widely used in the market at present, reads or writes data employing a semiconductor laser with a wave length of 650 nm and an objective lens with a numerical aperture (NA) of 0.6, whereas a BD employs a blue semiconductor laser with a wave length of 405 nm and an objective lens with the NA of 0.85, enabling an optical spot diameter very small.
Problems for the read/write techniques employing those optical systems are defocus, astigmatism caused by an inclined objective lens, coma-aberration caused by an inclined disk with respect to an optical axis, and a spherical aberration caused by the thickness deviation of cover layer and the like. These effects increase approximately proportional to the NA squared, the NA squared, a third power of the NA, and the fourth power of the NA, respectively, resulting in poor read/write. As for correcting the defocus and spherical aberrations, a technique of prior art employs, for example, as disclosed in JP Patent Publication (Kokai) No. 2001-222838A, a configuration wherein correction for the spherical aberration is made by varying the amount of correction, while holding the objective lens at a position wherein the amplitude of reproduced signal becomes maximum. The final amount of correction is determined and used as the one that produces the maximum amplitude of tracking error signal. Another technique disclosed in JP Patent Publication (Kokai) No. 2000-11388A, optimizes the distance between two group objective lenses, and at the same time, the off-set values in focus control operation based on the reproduced signals. Still another technique disclosed in JP Patent Publication (Kokai) No. 2005-100483A, utilizes the amplitude of tracking error signal to control spherical aberration and focal point position simultaneously, and then utilizes the reproduced signal amplitude to fine control the aberration and focal point position.